With the shader approach to symbolic recoloring, we must
not recolor the svgs anymore as we're loading them. Instead,
load them the same way that gtk-encode-symbolic-svg does.
This fixes the rendering of large symbolic icons e.g. the
'no search results found' page in the file chooser.
When creating icon info objects for unthemed files, we don't
really have a nominal size, so we pass 0 to mean 'load at
original size'. However, this is not what was happening.
To make this possible, add variants of some pixbuf loading
functions that take a scale factor instead of a desired size,
and use those when we don't have a nominal size.
